The current invention relates to a device for detecting objects in the vicinity of a vehicle by means of pulse radar, which has a transmitter that emits transmission pulses, and a receiver that evaluates reception pulses reflected against objects. A device of this kind, which is based on the known pulse radar principal, has been disclosed, for example, by DE-OS 28 35 867.
A pulse radar device can be used to very precisely determine distances from objects and therefore a pulse radar-based device is suitable for use in vehicles, e.g. as a parking aid, as a pre-crash sensor, or for detecting objects in the vicinity of the so-called blind spot. In all of these applications, the immediate vicinity of a vehicle is monitored for the presence of collision obstacles in this range and the distance from the vehicle to these obstacles is measured. Radar sensors that monitor the immediate vicinity of the vehicle are referred to as short-range radar. Particularly with a short-range radar, there is the problem that the signals reflected against objects at a very short distance lead to an overmodulation in the receiver. In addition, an internal crosstalk from the transmission side to the reception side and a resulting interference voltage in the receiver cannot be prevented. This crosstalk results in the fact that it is hardly possible to detect objects in a very short distance range. Because the travel time of a wave reflected against a very close object approximately coincides with the travel time of crosstalking signals so that the reflected reception signal is overlapped by the crosstalk signal. The measurement of signals that are reflected against objects in close proximity is consequently distorted.
The object of the invention, therefore, is to disclose a device of the type mentioned at the beginning, which can accurately detect even objects in close proximity to the vehicle and can likewise reliably detect more distant objects.
The stated object is attained with the features of claim 1 by virtue of the fact that means are provided, which can execute a selection of those reception pulses whose chronological offset in comparison to the respective transmission pulses is of such a magnitude that these reception signals result exclusively from reflections against objects from a selected distance range correlating to the chronological offset and that additional means are provided, which permit the transmission pulse power to be increased and decreased as the distance range rises and falls. Namely, if the transmission pulse power for the detection of objects in very close proximity to the vehicle is kept at a low level, then reflected signals do not lead to an overmodulation of the receiver and also, the crosstalk between transmitter and receiver is weakened to such an extent that reception pulses reflected against close objects can be accurately detected. If the power of the transmission pulses is increased in order to detect objects in a greater distance range, then even reception signals reflected against small objects are still powerful enough to be detected in the receiver.
Advantageous improvements of the invention ensue from the dependent claims. The selection of reception pulses is preferably executed by means of a mixer which is contacted on the one hand by the reception pulses received by an antenna and on the other hand, is contacted by reference signal pulses, which in comparison to the transmission pulses, have a time delay that corresponds to the selected distance range.
For the production of transmission pulses and reference signal pulses, pulse generators can be provided, which execute a pulse modulation of a high frequency signal carrier supplied by an oscillator. A very simple pulse modulation of the high frequency signal carrier is preferably executed by means of an electrically controllable switch, which is respectively provided for the transmission pulses and for the reference signal pulses and whichxe2x80x94controlled by the respective pulse generatorxe2x80x94switches the high frequency signal carrier through to a transmission antenna or to the mixer.
It is advantageous to derive the high frequency signal carrier for the transmission pulses and the high frequency signal carrier for the reference signal pulses from a single oscillator by means of a power divider.
The means, which are for controlling the transmission pulse power, control the pulse amplitude and/or the pulse width. The pulse width can be controlled, for example, by changing the opening times of the switches. The pulse amplitude of the transmission pulses can be controlled by means of a variable damping element or by changing the operating point of the oscillator.
An improved signal-to-noise ratio of the reception pulses can be achieved by the receiver executing an integration of a number of reception signals originating from a respectively selected distance range.